The spindle pole body (SPB) is the major microtubule organizing center of budding yeasts and is the functional equivalent of the centrosome in higher eukaryotes. We are using fast-frozen, freeze-substituted cells in conjunction with HVEM tomography to study the fine structure of the SPB and the events of early spindle formation. The SPB is organized from distinct layers that are physically coupled; it is anchored in the nuclear envelope membrane with hook-like structures. Individual microtubules (MTs) and their interactions within the nucleoplasm are imaged at 5-10 nm 3-D resolution, significantly better than that achieved by conventional electron microscopy. MT minus ends are capped and are tethered to the SPB inner plaque, whereas the majority of nuclear MTs have a distinct flaring at their plus ends. Unbudded yeast cells containing a single SPB retain 16 MTs, enough to attach to each of the expected 16 chromosomes. Their median length is ~150 nm. MTs growing from duplicated but not separated SPBs have a median length of ~130 nm and interdigitate over the bridge. As a bipolar spindle is formed the median MT length increases to ~300nm, then decreases dramatically to ~30 nm in cells in late anaphase. 3-D models confirm that there is no conventional metaphase and that anaphase A occurs. These studies complement and extend what is known about the 3-D structure of the yeast mitotic spindle by furthering our understanding of the organization of the SPB in intact cells (See Research Highlight #3).